This invention relates generally to magnetically tuned resonant circuits, and more particularly to low noise magnetically tuned resonant circuits.
As it is known in the art, magnetically tuned resonant circuits, such as YIG filters, are used in many radio frequency applications, such as radar receivers. One application for a magnetically tuned resonant circuit is in a radio frequency oscillator. In particular, one type of oscillator includes a YIG band pass filter disposed in the feedback circuit of an amplifier. When the open loop gain and phase conditions of the oscillator are satisfied simultaneously at a certain frequency that is, when the open loop gain is greater than unity and the open loop phase shift is equal to an integer multiple of 2.pi. radians, the circuit will operate as an oscillator at that particular frequency. A second application for a magnetically tuned resonant circuit is as a dispersive element in an interferometer type of frequency discriminator. For example, a microwave voltage controlled oscillator (VCO) which typically produce signals with high levels of frequency modulation (FM) noise, is stabilized with a frequency lock loop using the YIG filter as the dispersive element in the frequency discriminator.
In many of these applications the noise performance of the oscillator is a very important consideration. For example, in a doppler radar, noise generated at baseband frequencies that is noise generated at the frequencies of the order of expected doppler frequency shifts will reduce the subclutter visibility of the radar. In each of the applications mentioned above, the YIG filter or the magnetically tuned resonant circuit contributes to the noise induced in the circuit. This contribution is particularly important when the other components in the particular circuit are low noise components. Therefore, it is desirable to provide microwave tunable oscillators having very low noise characteristics.